1. Field of the Invention
This invention relates to an oxygen concentration detector for detecting the oxygen concentration of a gas, which detector is particularly suitable for detecting the oxygen concentration of a gas at a low temperature.
2. Description of the Prior Art
To measure the oxygen concentration in exhaust gas from internal combustion engines, oxygen concentration detectors made of oxygen-ion-conductive solid electrolyte and based on the principle of an oxygen concentration cell have been used. A typical example of such oxygen concentration detectors uses yttria-added zirconia porcelain as the solid electrolyte with platinum electrodes mounted thereon.
The oxygen concentration detectors of the prior art have shortcomings in that when the temperature of the oxygen concentration detector is low, the catalytic ability of platinum is reduced while the electric resistance of the solid electrolyte is increased and the impedance of the oxygen concentration detector becomes high, and the reduced catalytic ability and the high impedance tend to render the oxygen concentration detector susceptible to adverse effects of noises or the like and to reduce the response speed thereof, and the lowest temperature limit for practical use of the oxygen concentration detector has been about 350.degree. C. On the other hand, the temperature of the exhaust gas from internal combustion engines can be below the above-mentioned lowest temperature limit at the start or during slow running of the engines, so that the conventional oxygen concentration detectors have not been used to their full capability.
To solve this problem, it has been proposed, for instance, to insert a coiled heater wire in a cylindrical solid electrolyte with a bottom, so as to heat the solid electrolyte by the heater wire. However, the use of the heater wire has shortcomings in that the structure becomes complicated, that a large power of up to several tens of watts is necessary, and that the heater wire becomes thin as being used over a long period of time and sometimes the heater wire is broken. The oxygen concentration detectors of the prior art are used to detect the condition that the value of the air-fuel ratio .lambda. of an internal combustion engine is 1.0. In order to measure a specific value of the air-fuel ratio .lambda. which is different from 1.0, it is not sufficient to accurately measure the electromotive force of an oxygen concentration cell, but the temperature thereof must be measured accurately. To this end, for instance, it has been proposed to dispose a temperature sensing element in a cylindrical solid electrolyte with a bottom for measuring the temperature of the solid electrolyte. However, the use of such temperature sensing element has shortcomings in that the temperature of the solid electrolyte exposed to the exahust gas from an internal combustion engine is not uniform and the temperature measurement at one point thereof does not provide accurate picture of the solid electrolyte temperature; that when the temperature of the exhaust gas is changed the output of the temperature sensing element has a time lag in detecting the temperature change of the solid electrolyte so that the temperature of the solid electrolyte cannot be measured accurately; that the structure of the oxygen concentration detector is complicated.
If the coiled heater wire and the temperature detecting element are inserted in the solid electrolyte for effecting both the heating and the temperature detecting, the structure of the oxygen concentration detector becomes further complicated and unpracticable.